This replaces a helper used in tests. The intention is to use it from other tests, and seemed like a good fit to place it under the KMS connector object itself. Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2159>
As other KMS tests, depends on being DRM master and vkms being loaded. Currently consists of a sanity check that checks for the expected set of connectors, CRTCs, planes, etc. Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2159>